Cyanoacrylate adhesive

ABSTRACT

Cyanoacrylate adhesives which have increased heat resistance, yet also exhibit good storage stability and acceptable cure times, are obtained by using 2-cyanopentadienoic acid esters in combination with alkylene bis-(2-cyanopentadienoates).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a cyanoacrylate adhesive containing2-cyanopentadienoic acid ester and to its production and use.

2. Discussion of Related Art

Cyanoacrylate adhesives are one-component reactive adhesives based onmonomeric 2-cyanoacrylates. They have cornered the market throughextremely quick curing which, depending on the substrate, takes only afew seconds. The resulting properties satisfy many industrialrequirements. However, there are special applications where they arestill in need of improvement. For example depolymerization is possibleat temperatures of only around 100° C. so that prolonged exposure totemperatures above 80° C., should be avoided.

This problem of inadequate heat stability has been repeatedly discussed,for example in Journal of Applied Polymer Science 49, (1993), pages1387-1394. This literature reference first of all describes theproduction and characterization of alkyl-2-cyano-2,4-pentadienoates andalkyl-2-cyano-2,4-hexadienoates and of bis-(2-cyano-2,4-hexadienoates)and then investigates the effect of adding 10% by weight of individualalkyl esters on the thermal stability of polycyanoacrylic acid propylester. The results show that heating for 1 hour to 150° C. does not havean adverse effect on strength at 25° C. In the absence of such anaddition, however, there is a reduction in strength to about one quarterof the original value. There is no reference to strength at elevatedtemperature, nor any indication of how it can be improved.

WO 94/15907 describes cyanoacrylate adhesives with improved resistanceto heat and moisture through the use of cyanoacrylate-terminated linearpolymers, for example polyisobutylene or polyalkyl methacrylates (seepage 4, line 16 to page 5, line 7 and page 10, line 27 to page 11, line12). There are no specific references either to the composition of theadhesives or to their adhesive properties.

The fact that an addition of biscyanoacrylates to cyanoacrylateadhesives can lead to problems, particularly in regard to storage lifeand curing rate, is well known. In Journal of Polymer Science: PolymerChemistry Edition, Vol. 16, pages 2475-2507 (1978), Carl J. Buckdescribes a mixture of isobutyl cyanoacrylate andbis-(2-cyanoacrylates). N,N-dimethyl-p-toluidine (DMPT) is required toinitiate the polymerization reaction. A composition of 90% by weight ofmethyl cyanoacrylate and 10% by weight of octanediol bis-cyanoacrylatetakes one day to cure at 38° C. (see page 2504, last paragraph, and page2505).

Against the background of this prior art, the problem addressed by thepresent invention was to provide a cyanoacrylate adhesive which wouldshow increased heat resistance for substantially the same storagebehavior and acceptable cure times.

SUMMARY OF THE INVENTION

The solution provided by the present invention is defined in the claimsand is essentially distinguished by the fact that, in addition to atleast one typical monocyanoacrylate, the cyanoacrylate adhesivecontaining 2-cyanopentadienoic acid alkyl ester (CPDS-alkyl) alsocontains an effective quantity of at least one alkylenebis-(2-cyanopentadienoate) (bis-CPD).

DETAILED DESCRIPTION OF THE INVENTION

In the context of the invention, “typical monocyanoacrylates” areunderstood to be compounds corresponding to the following generalformula:

H₂C═C(CN)—CO—O—R  (I)

where R is an alkyl, alkenyl, cycloalkyl, aryl, alkoxyalkyl, aralkyl orhaloalkyl group, more especially a methyl, ethyl, n-propyl, isopropyl,n-butyl, isobutyl, pentyl, hexyl, allyl, methallyl, crotyl, propargyl,cyclohexyl, benzyl, phenyl, cresyl, 2-chloroethyl, 3-chloropropyl,2-chlorobutyl, trifluoroethyl, 2-methoxyethyl, 3-methoxybutyl and2-ethoxyethyl group. The cyanoacrylates mentioned above are known to theexpert on adhesives, cf. Ullmann's Encyclopaedia of IndustrialChemistry, Vol. A1, page 240, Verlag Chemie Weinheim (1985) and U.S.Pat. Nos. 3,254,111 and 3,654,340. Preferred monomers are the allyl,methoxyethyl, ethoxyethyl, methyl, ethyl, propyl, isopropyl or butylesters of 2-cyanoacrylic acid. The monocyanoacrylates represent thelargest percentage by weight of polymerizable monomers in the adhesive.

The abbreviation “bis-CPDs” is understood to apply to compoundscorresponding to the following general formula:

[CH₂═CH—CH═C(CN)—CO—O]₂R¹  (II)

where R¹ is a branched or unbranched difunctional alkylene radicalcontaining 2 to 18 and more particularly 2 to 4 carbon atoms which mayalso contain hetero atoms, such as halogens and oxygen. However, R¹ ispreferably a pure hydrocarbon. It is important that the bis-CPDs shouldbe particularly pure. This requirement is satisfied, for example, by thefollowing production and purification methods. Basically, thecyano-pentadienoates may be obtained by the Gerber method (A. H. Gerber,U.S. Pat. No. 3,316,227 (1967)). However, the compounds are obtained ingreater purity if, in contrast to the method mentioned above, thereaction mixture is taken up in 4 to 5 times the volume of ca. 5%hydrochloric acid and the precipitate formed is recrystallized frompetroleum ether after filtration and drying in air. After drying in ahigh vacuum, the yields are generally of the order of 50 to 70%. Theadhesive formulations prepared with a product obtained in this way aredistinguished by greater reactivity, i.e. faster setting times, and alsoby better stability in storage.

In addition, the required CPDS-alkyl/bis-CPD mixture can be obtained bya one-pot synthesis, i.e. by simultaneous reaction of the correspondingcyanoacetates in one and the same batch.

The abbreviation “CPDS-alkyl” is understood to apply to compoundscorresponding to the following general formula:

CH₂═CH—CH═C(CN)—CO—O—R²  (III)

where R² is a branched or unbranched monofunctional alkyl radicalcontaining 1 to 10 carbon atoms and more particularly 1 to 4 carbonatoms which may also contain hetero atoms, such as halogens and oxygen.However, R² is preferably a pure hydrocarbon. It is important that theCPDS-alkyls should be particularly pure. This requirement is satisfied,for example, by the production and purification method described above.

The bis-CPDs and CPDS-alkyls in pure form can be stored in light forabout 4 weeks at room temperature without significantly discoloring orundergoing changes in their melting points. In a refrigerator, i.e. at T<10° C./darkness, they can be stored for at least 6 months. However,they can presumably be stored for much longer.

The compounds are stable to extremely weak bases and nucleophiles, forexample water. In the presence of medium-strong to strong bases, forexample amine bases, phosphines, lyes, they undergo spontaneouspolymerization similarly to the cyanoacrylates.

The 2-cyanopentadienoates (CPDS-alkyl and bis-CPD) should preferably beadded in a quantity of up to 25% by weight and more preferably in aquantity of 1 to 15% by weight, based on the total weight of theadhesive formulation.

The ratio by weight of CPDS-alkyl to bis-CPD can vary within widelimits, although it should preferably be in the range from 4 to 0.25:1and more preferably in the range from 2 to 0.5:1.

The adhesive may contain additives, for example plasticizers,thickeners, stabilizers, primers, dyes etc. The adhesive preferablycontains stabilizers such as, for example, hydroquinone and sulfurdioxide.

The adhesive is prepared in the usual way by mixing the components,preferably by dissolving the respective additives at room temperature.

In all the cases investigated, the stability of the new adhesives instorage exceeded 3 months at room temperature or 3 days at 60° C.

The new cyanoacrylate adhesive according to the invention isparticularly suitable for bonds which have to satisfy stringent thermalrequirements, for example the bonding of electrical and electroniccomponents.

The Invention is Illustrated by the Following Examples

I. Starting Compounds and Their Preparation

1. 2-Cyanopentadienoic acid methyl ester=CPDS-Me

2. 2-Cyanopentadienoic acid ethyl ester=CPDs-Et

3. Tetramethylene-bis-(2-cyanopentadienoate)=TMBCP

4. Ethylene-bis-(2-cyanopentadienoate)=bis=CPD

5. Ethyl cyanoacrylate

6. Isopropyl cyanoacrylate

Comonomers 1 to 4 were prepared by the method described above.

The comonomers were characterized by CHN analysis, IR spectroscopy and¹H-NMR.

II. Composition and Production of the Cyanoacrylate Adhesive

The composition in respect of the monomers is shown in Tables 1 and 2.The addition of comonomers in % by weight is based on the final adhesiveformulation.

In addition to the monomers, the adhesive contains the followingadditives:

1. 500 ppm hydroquinone

2. 10 ppm sulfur dioxide.

The cyanoacrylate adhesive according to the invention can be preparedfrom these components in the usual way. Preferably, all the componentsof the adhesive are dissolved at room temperature by shaking orstirring.

The adhesive has typical viscosities of about 2.2 to 4.0 mPas (at20±0.1° C., as measured in accordance with DIN 54453). Their storagelife is also within the usual limits. All the samples prepared werestill liquid after storage for 3 months at room temperature.

III. Adhesive Properties

1. Contact Time

The contact time is the setting time on EPDM foam rubber, i.e. theperiod of time for which the parts to be joined have to be pressedtogether until material failure occurs during separation of the bond.

2. Tensile Shear Strength

The test specimens were produced and prepared as follows:

Cleaned and sandblasted Al plates measuring 25×100 mm were bonded with a12.5 mm overlap. The specimens were then stored first for 5 days in aconditioned atmosphere (20° C./65% relative air humidity) and then for 2days at 105° C. in a heating cabinet.

The tensile shear strength was measured at the indicated temperature inaccordance with DIN EN 1465 (rate of advance 10 mm/min.) and isexpressed in N/mm².

The results are set out in Tables 1 and 2. The tests show that thecontact time is only negligibly lengthened and, in all the casesinvestigated, is between 5 and 6 seconds. Both monofunctional andbifunctional additives improve tensile shear strength at elevatedtemperature. In the cases mentioned (see Table below), the valuesobtained where the compounds are used in combination are distinctlyhigher than might have been expected from the individual effect's takentogether. Accordingly, a synergistic effect may be assumed to occur inthe Examples mentioned.

TABLE 1 Tensile shear strength and contact time in dependence upon thetype and composition of the monomers Basic Monomers added Tensile shearContact mon- Type/propor- Type/propor- strength time No. omer tion %tion % N/mm² ° C. secs.  1 Ethyl CPDS-ME/0 Bis-CPD/0 3.6 105 3 cyano-acrylate  2 5 0 6.8 5-6  3 8 0 6.1 5-6  4 11 0 5.9 5-6  5 0 5 6.4 5-6  65 5 10.5 5-6  7 CPDS-ME/0 Bis-CPD/0 1.3 130  8 5 0 2.4  9 8 0 1.3 10 110 1.8 11 0 5 3.1 12 5 5 8.1 13 CPDS-ET/0 Bis-CPD/0 1.3 130 14 0 5 3.2 15Ethyl CPDS-ET/5 Bis-CPD/0 3.1 cyano- acrylate 16 5 5 6.8 17 ″ CPDS-ME/0TMBCP 0 1.3 130 18 0 5 2.9 19 5 0 2.4 20 5 5 4.6 21 Iso- CPDS-ET/0 TMBCP0 2.6 130 propyl cyano- acrylate 22 0 0.2 2.8 23 0 1.2 3.4 24 5 0.2 5.525 5 1.0 7.8 26 5 0 4.8

What is claimed is:
 1. A cyanoacrylate adhesive comprised of one or more2-cyanopentadienoic acid esters and one or morealkylene-bis-(2-cyanopentadienoates).
 2. The cyanoacrylate adhesive ofclaim 1 wherein at least one of the alkylene-bis-(2-cyanopentadienoates)has an alkylene group containing 2 to 18 carbon atoms.
 3. Thecyanoacrylate adhesive of claim 1 wherein at least one of the2-cyanopentadienoic acid esters has an alkyl group containing 1 to 10carbon atoms.
 4. The cyanoacrylate adhesive of claim 1 wherein the2-cyanopentadienoic acid esters and thealkylene-bis-(2-cyanopentadienoates) comprise in total up to 25 percentby weight of the cyanoacrylate adhesive.
 5. The cyanoacrylate adhesiveof claim 1 characterized by a ratio by weight of 2-cyanopentadienoicacid esters to alkylene-bis-(2-cyanopentadienoates) of 4:1 to 0.25:1. 6.The cyanoacrylate adhesive of claim 1 additionally comprising one ormore monocyanoacrylates corresponding to the general formulaH₂C═C(CN)—CO—O—R where R is an alkyl, alkenyl, cycloalkyl, aryl,alkoxyalkyl, aralkyl or haloalkyl group.
 7. The cyanoacrylate adhesiveof claim 6 wherein at least one of said monocyanoacrylates is selectedfrom the group consisting of allyl, methoxyethyl, ethoxyethyl, methyl,ethyl, propyl, isopropyl and butyl esters of 2-cyanoacrylic acid.
 8. Thecyanoacrylate adhesive of claim 6 wherein the monocyanoacrylatesrepresent the largest percentage by weight of polymerizable monomers inthe cyanoacrylate adhesive.
 9. The cyanoacrylate adhesive of claim 1additionally comprising one or more additives selected from the groupconsisting of plasticizers, thickeners, stabilizers, primers, and dyes.10. A method of bonding an electrical or electronic component comprisingusing the cyanoacrylate adhesive of claim 1 to form an adhesive bond insaid electrical or electronic component.
 11. A cyanoacrylate adhesivecomprised of (a) one or more monocyanoacrylates corresponding to thegeneral formula H₂C═C(CN)—CO—O—R where R is an alkyl, alkenyl,cycloalkyl, aryl, alkoxyalkyl, aralkyl or haloalkyl group; (b) one ormore 2-cyanopentadienoic acid esters having an alkyl group containing 1to 10 carbon atoms; and (c) one or morealkylene-bis-(2-cyanopentadienoates) having an alkylene group containing2 to 18 carbon atoms; wherein the 2-cyanopentadienoic acid esters andthe alkylene-bis(2-cyanopentadienoates) comprise in total up to 25percent by weight of the cyanoacrylate adhesive and wherein thecyanoacrylate adhesive is characterized by a ratio by weight of2-cyanopentadienoic acid esters to alkylene-bis-(2-cyanopentadienoates)of 4:1 to 0.25:1.
 12. The cyanoacrylate adhesive of claim 11 wherein thealkylene group contains 2 to 4 carbon atoms.
 13. The cyanoacrylateadhesive of claim 11 wherein the alkyl group containing 1 to 4 carbonatoms.
 14. The cyanoacrylate adhesive of claim 11 wherein the ratio of2-cyanopentadienoic acid esters to alkylene-bis-(2-cyanopentadienoates)is from 2:1 to 0.5:1.
 15. The cyanoacrylate adhesive of claim 11 whereinthe 2-cyanopentadienoic acid esters and thealkylene-bis-(2-cyanopentadienoates) comprise in total from 1 to 15percent by weight of the cyanoacrylate adhesive.
 16. The cyanoacrylateadhesive of claim 11 wherein at least one of the monocyanoacrylates isselected from the group consisting of allyl, methoxyethyl, ethoxyethyl,methyl, ethyl, propyl, isopropyl and butyl esters of 2-cyanoacrylicacid.
 17. The cyanoacrylate adhesive of claim 11 additionally comprisingone or more additives selected from the group consisting ofplasticizers, thickeners, stabilizers, primers, and dyes.
 18. Acyanoacrylate adhesive comprised of (a) one or more monocyanoacrylatesselected from the group consisting of allyl, methoxyethyl, ethoxyethyl,methyl, ethyl, propyl, isopropyl and butyl esters of 2-cyanoacrylicacid; (b) one or more 2-cyanopentadienoic acid esters having an alkylgroup containing 1 to 4 carbon atoms; and (c) one or morealkylene-bis-(2-cyanopentadienoates) having an alkylene group containing2 to 4 carbon atoms; wherein the 2-cyanopentadienoic acid esters and thealkylene-bis-(2-cyanopentadienoates) comprise in total from 1 to 15percent by weight of the cyanoacrylate adhesive and wherein thecyanoacrylate adhesive is characterized by a ratio by weight of2-cyanopentadienoic acid esters to alkylene-bis-(2-cyanopentadienoates)of from 2:1 to 0.5:1.
 19. The cyanoacrylate adhesive of claim 18 whereinat least one of the 2-cyanopentadienoic acid esters is selected from thegroup consisting of 2-cyanopentadienoic acid methyl ester and2-cyanopentadienoic acid ethyl ester.
 20. The cyanoacrylate adhesive ofclaim 18 wherein at least one of thealkylene-bis-(2-cyanopentadienoates) is selected from the groupconsisting of ethylene-bis-(2-cyanopentadienoate) andtetramethylene-bis-(2-cyanopentadienoate).
 21. The cyanoacrylateadhesive of claim 18 additionally comprising one or more additivesselected from the group consisting of plasticizers, thickeners,stabilizers, primers, and dyes.
 22. A method of bonding electrical orelectronic component comprising using the cyanoacrylate adhesive ofclaim 18 to form an adhesive bond in said electrical or electroniccomponent.